The invention relates to a pneumatic door-operating arrangement with an electropneumatic control device, especially for doors in vehicles which are used for transporting people.
Door operation installations are, as a rule, controlled pneumatically or electropneumatically, i.e., to open or close the door the driver triggers a pneumatic or electrical pulse, which, by means of a control valve (also called a door valve), acts on a door cylinder (also called door drive) with compressed air, whereby the stroke movements of the door cylinder piston trigger the opening and closing movements of the door.
A general requirement of such an installation is that a closing door automatically switch into an opening movement if persons or objects are caught in the door, or that the installation automatically become depressurized if persons or objects are caught in an opening door.
There is an additional requirement that when a door operation device which has been vented by means of an emergency valve is re-pressurized, it does not produce a sudden movement of the door, so that there are no damages to the door or injuries to persons.
A pneumatic door operation device of the type described above, which fulfills the safety requirements mentioned above, is known from the unpublished German Patent Application No. P 31 18 764.1 (copending U.S. application Ser. No. 386,550). In this installation, the functions of a door valve and the valve apparatus which produces the safety functions described above are combined in one valve combination. However, one disadvantage with this installation is that when there is a break in the electrical cable system, or a failure of magnetic forces, there can be an undesirable drop in the installation pressure. Even an inadvertent extraction of the ignition key can cause such a pressure drop. This disadvantage results from the fact that the door must be opened and held open by means of a constant electrical current, and that, for this reason, the corresponding pressurizing valve must be opened or held open against the force of a spring. When the electrical current fails, the valve element of the pressurizing valve returns to its original venting position under the force of the spring, so that there can be a pressure drop in the above-mentioned door operation installation.